1. Field of the Invention
The present invention relates to a luminescent device which can emit light by converting electric energy into light, and in particular to an organic electroluminescent device.
2. Description of the Related Art
Organic electroluminescent devices (hereinafter sometimes referred to as organic EL devices or simply as luminescent devices) can give light of high luminance at low voltage, and are thus noteworthy as a promising display device. The external quantum efficiency is an important property value of this organic electroluminescent device. The external quantum efficiency is calculated according to the equation below, and the device is considered to be more advantageous with regard to energy consumption if the value is larger, that is, if more photons are released per electron injected into the device.
External quantum efficiency φ=number of photons released from the device/number of electrons injected into the device.
The external quantum efficiency of the organic electroluminescent device is determined specifically according to the following equation:External quantum efficiency φ=internal quantum efficiency×light extraction efficiency.
In organic EL devices utilizing fluorescent emission from organic compounds, the upper limit of internal quantum efficiency is 25%. Since the light extraction efficiency is about 20%, the upper limit of external quantum efficiency is estimated to be about 5%.
For improving the external quantum efficiency of an organic electroluminescent device by improving the internal quantum efficiency of the device, a device using a triplet luminescent material (phosphorescent material) has been reported (see, for example, International publication No. WO 2000/070655). This device can achieve a higher external quantum efficiency than a conventional device (singlet luminescent device) utilizing florescent emission, and can realize a maximum external quantum efficiency of 8% (an external quantum efficiency at 100 cd/m2 is 7.5%). However, since phosphorescent emission from a heavy atom metal complex is used in the device using a phosphorescent material, the emission response is slow, and there are also needs for improvement in durability.
In order to solve this problem, a singlet luminescent device using energy transfer from triplet exciton to singlet exciton has been reported (see, for example, WO 2001-008230). However, the maximum external quantum efficiency of the device described in this reference is as low as 3.3%, and does not exceed the external quantum efficiency (φ=5%) of the conventional singlet luminescent device. Therefore, there are needs for further improvement in efficiency.